1. Field of the Invention
The present invention relates to a mass flow meter and a mass flow controller.
2. Description of the Prior Art
For example, in the case where an apparatus for producting semiconductors is supplied with various kinds of gas used for producing semiconductors, their supply passages are provided with a mass flow controller, respectively, to control flow rates of the respective gases.
FIG. 5 schematically shows a construction of a control system of the general mass flow controller 50. Referring to FIG. 5, reference numeral 51 designates a flow rate-measuring portion comprising a thermal flow rate sensor 54 provided in a measuring passage 53, a bridge circuit 55 and an amplification circuit 56 for measuring a flow rate of a gas G passing through a passage 52 to put out a flow rate signal (x) corresponding to said flow rate of said gas G. And, reference numeral 57 designates a by-pass passage provided so as to bypass said measuring passage 53. Said by-pass passage 57 is provided with a by-pass portion 58 having constant-flow rate-ratio characteristics. In addition, reference numeral 59 designates a control valve provided on the downstream side of the confluence of the measuring passage 53 and said by-pass passage 57. Reference numeral 60 designates a comparison circuit for comparing said flow rate signal (x) put out from said flow rate-measuring portion 51 with a flow rate-setting signal (y) from a flow rate-setting portion (not shown), reference numeral 61 designating a control circuit, and reference numeral 62 designating a control valve-driving circuit.
And, with the mass flow controller 50 having the above described construction, when the flow rate signal (x) is larger than said flow rate-setting signal (y), an instruction for reducing an openness of said control valve 59 is given from said control circuit 61, while, when the flow rate signal (x) is smaller than the flow rate-setting signal (y), an instruction for increasing said openness of the control valve 59 is given from the control circuit 61, so as to supply an apparatus (not shown) provided on the downstream side of the mass flow controller 50 with an appointed flow rate of gas G.
However, in the conventional mass flow controller 50, in order to prevent an excessive flow rate of gas G from passing through the measuring passage 53, the control valve 59 of small flow rate has been incorporated. The openness of the control valve 59 in the mass flow controller, of which full-scale flow rate is for example 5 ml/min, has been set so that the gas G may pass at a flow rate of merely about 7 to 8 ml/min when a differential pressure is 0.5 kg/cm.sup.2 G. And, even though said differential pressure was 3.0 kg/cm.sup.2 G, the gas G could be passed at a flow rate of merely about 50 ml/min.
Accordingly, in the mass flow controller 50 with the control valve 59 of small flow rate incorporated therein, as above described, it has taken a long time to conduct a formation of a vacuum in a gas line, a displacement of an inside of said gas line with an inert gas and the like.
On the contrary, a by-pass line 63 has been provided in parallel to a mass flow controller 50, as shown by for example an imaginary line in FIG. 5, but, in this case, a disadvantage has occurred in that the whole construction is complicated and large-scaled.
So, the above described problems can be solved by incorporating a control valve, which can pass a gas at a sufficiently large flow rate, in a mass flow controller of small flow rate.
However, in the case where said control valve, which can pass a gas at a sufficinetly large flow rate, is incorporated in said mass flow controller of small flow rate under the conventional technical condition, the following problems occur anew.
That is to say, flow-rate output characteristics of a general flow rate-measuring portion 51 as shown in FIG. 5 produce an inversion of phenomenon when a flow rate exceeds an appointed limit, as shown in FIG. 6. In FIG. 6, an axis of abscissa shows said flow rate of gas (fluid) passing through said mass flow controller 50 while an axis of ordinate shows an output from said flow rate-measuring portion 51.
And, the mass flow controller 50 having such the characteristics is usually used within a normal operation range N encircled by an imaginary line in FIG. 6, but, when an excessive flow rate of gas G exceeding said normal operation range N is suddenly passed through the mass flow controller 50, said output is reduced in spite of said excessive flow rate of gas G in the case where for example an internal volume of a gas passage 52 from a thermal flow rate sensor 54 to a control valve 59 is large or a response speed of said control valve 59 is small. Accordingly, although a signal for closing the control valve 59 is ought to be originally put out from a control circuit 61, contrarily a signal for further opening said control valve 59 is put out from said control circuit 61 and thus the mass flow controller 50 is fallen into an abnormal operating condition when for example the flow rate exceeds Q.sub.x in FIG. 6.
Such the problems have occurred also in a mass flow meter having a construction almost same as that of the above described mass flow controller 50 from which the control valve 59 and a member for opening and closing the control valve 59 are removed. That is to say, in this mass flow meter, a value of flow rate indicated has been reduced in spite of an excessive flow rate of gas G and thus an indication corresponding to the real flow rate has not been given.